Apparatus for controlling mist and dust in the manufacture and finishing of paper and board

ABSTRACT

The present invention relates to a method and assembly for controlling the behaviour of particulate matter such as mist and dust occurring in the manufacture of paper and board. In the method, a web ( 1 ) is passed to web treatment equipment ( 22, 23 ) wherein at least one treatment step is applied to the web ( 1 ) causing the emission of said particulate matter ( 24 ). According to the method, in the vicinity of the emission point ( 21 ) of said particulate matter are placed at least two electrodes ( 25, 26 ) and at least one electrode, called a counter-electrode ( 26 ), is taken to a low potential. Further, at least one electrode ( 25 ) is taken to a potential higher than that of said counter-electrode ( 26 ), and the potential difference between said electrodes ( 25, 26 ) is made so high as to generate a corona discharge between said electrodes that causes an ion-blast wind toward the electrode ( 26 ) of the lower potential, said ion-blast wind being capable of transferring particulate matter, which enters the gap between said electrodes, toward said electrode of lower potential.

[0001] The present invention relates to a method according to thepreamble of claim 1 for collecting water vapour, escaped fibrous matter,coat mist and dry fibrous dust occurring in the manufacture of paper andboard or adhering the same to the web being processed.

[0002] The invention also concerns an assembly for implementing saidmethod.

[0003] The manufacture of paper and board can be divided into two phasescomprising the formation of the base web, that is, the base board orpaper, followed by the treatment of this web by coating, calendering,slitting and rolling. With the exception of rolling, all of these stepsinvolve emission of different kinds of mist and dust that aredetrimental if allowed to escape to the surroundings. Hence, a majorportion of the paper machine and finishing equipment must be enclosed inhoods and enclosures. From the interior of such enclosed spaces, themists are removed by suction with a vacuum. The air sucked off from theenclosed spaces is cleaned from moisture, dust and mist prior todischarging the air into the ambient atmosphere. Such enclosures as wellas the cleaning of the sucked air are expensive to implement. Thecollection systems must be designed for large volumetric air flows,because the internal surfaces of the hoods and equipment must be keptfree from condensation or dirt that could fall or otherwise land on theweb being manufactured.

[0004] Particularly in coating a paper web, water droplets or otherforeign matter falling on the web can easily cause defects in the webbeing made. However, regions of insufficient flow velocity may remain inthe hood structures that thus may allow accumulation of foreign matterin the system. Furthermore, the collection of coating mist in particularfrom the exhaust air is cumbersome and complex to arrange, because thecollected waste coat cannot be dumped in the plant sewer system due toeconomical and environmental reasons, but rather, it is recirculatedback to the machine circulation. However, as the coat returned to themachine circulation must be free from foreign matter and air bubbles,the collection of coat mist from an air flow is a technicallychallenging task.

[0005] It is an object of the present invention to provide a methodsuited for controlling the emissions of dusts, mists and other matteroccurring in droplet or particle form in paper manufacture by virtue ofguiding such matter to desired surfaces.

[0006] The goal of the invention is achieved by way of guiding thedroplets or particles to be treated to impinge on a desired surface bymeans of both an electric field imposed between a counter-electrodetaken to a low potential, advantageously to the ground potential, and aplurality of electrodes, advantageously having a pointed structureswhich are taken to an elevated potential, and additionally by theion-blast wind induced by a corona discharge generated in the vicinityof the electrodes taken to the elevated potential. Such a collectingsurface may be formed by, e.g., the web being made or, alternatively, apurpose-made ground-potential electrode.

[0007] More specifically, the method according to the invention ischaracterized by what is stated in the characterizing part of claim 1.

[0008] Furthermore, the assembly according to the invention ischaracterized by what is stated in the characterizing part of claim 8.

[0009] The invention offers significant benefits.

[0010] By virtue of the invention, air-borne foreign matter can becollected directly to a desired surface which may be a web being treatedor formed, for instance. In the case that the coat dust, oralternatively, the humid and fiber-containing mist emitted from the webbeing formed can be effectively returned to the web running in theprocess, the amount of foreign material to be removed by means of avacuum will be reduced substantially and the cleaning of the exhaust airbecomes easier. The mist emitted from the coating equipment can becollected directly to a counter-electrode (ground electrode), wherebythe collection and removal of coating mist takes place in a single step.Since the mist adheres under electric forces to the collectingelectrode, the coating mix layer thus formed contains less air thanwaste coating collected by conventional techniques from an air flowmaking the collected coat easier to return to the machine circulation.The design of the assembly is readily modifiable which is a greatbenefit as the installation space available in paper machines isextremely limited due to different reasons. By virtue of the assemblyaccording to the invention, the emissions to be collected can becaptured very close to their point of origin that helps to prevent thesoiling of the paper-making equipment. A particularly advantageousbenefit is the possibility of returning the collected material back tothe web, whereby the amount of recirculating material is reduced.

[0011] In the following, the invention will be examined in greaterdetail by making reference to the appended drawings, in which

[0012]FIG. 1 shows schematically an embodiment of the invention; and

[0013]FIG. 2 shows schematically an embodiment of the invention.

[0014] The function of the present invention is based on an applicationof the so-called ion-blast technique. In this method, a strong electricfield is established between one or generally a number of pointeddischarge electrodes and a planar counter-electrode. The tip of thepointed electrode supports a corona discharge that charges particles inthe vicinity of the electrode and causes formation of ions in theelectronegative gas. The ions migrate along the flux lines of theelectric field formed between the discharge electrode and theground-potential counter-electrode, whereby the ions adhere to particleson which they impinge on their travel. The electric field transportsparticles thus charged to the ground-potential electrode on which theyadhere by electric and mechanical forces. If the spacing between theelectrodes is made long and the voltage sufficiently high (greater than50 kV), a gas flow will be created capable of mechanically transferringtoward the ground-potential electrode the particles which are passingbetween the electrodes. This phenomenon is called the ion-blast wind. Inthe ion-blast effect, the electric field formed from the tip of apointed electrode will create at the electrode tip a conical field inwhich the ionized gas and particles are transported. The ion-blasteffect will affect both solid particles and liquid droplets.

[0015] In paper-making, the ion-blast effect can be utilized for bindinga raw material to a web formation substrate or a collecting platformfrom which the collected material can be removed using a suitabletechnique. In practice, the collecting substrate may be formed by anysurface which is transparent to the electric field or, alternatively, isa conductive surface. As the web in a continuous process is formed on amoving surface, the formation substrate is generally a wire, felt orband. When the method is employed for collecting material at a coatingstation, for instance, a counter-electrode may be used as the collectingsubstrate.

[0016] In FIG. 1 are shown different applications of the invention. Thefirst embodiment illustrated in the diagram is particularly suited foradhering dust or coat mist to a web. Herein, a web 1 travels supportedby four guide rolls 2-5. The first guide roll 2 and the last guide roll5 only serve to support the incoming and outgoing web 1, respectively.Over guide rolls 3, 4, which are adapted between the outer guide rolls2, 5, is passed a conductive wire 6 which is arranged to travel along aclosed triangular path so as to run over said web-supporting rolls 3, 4and a wire guide roll 7 which is mounted at a distance from the web 1itself. The rolls 3, 4, 7 guiding the conductive wire 6 are taken to theground potential thus allowing said conductive wire 6 to provide aground-potential surface under the web 1 running on said wire. On theopposite side of the web 1 in regard to the conductive wire 6, there arearranged pointed electrode tips 8 which are taken to an elevatedpotential and are mounted on an electrode support frame 10. Theelectrode support frame 10 is connected to a high-voltage supply 11.Next to the electrode tips 8 on the machine-direction travel of the web1 is mounted a post-corona device 12 that is also connected to thehigh-voltage supply 12.

[0017] Further next on the travel of the web 1 is adapted apost-collector unit 13 serving to remove from the web 1 the dust justtransferred to said web. The post-collector unit 13 comprises anenclosure housing a counter-electrode 15, whereby said enclosure istaken by means of a fan 14 to a vacuum and has its open side adapted toface the web 1. As the function of the post-collector unit 13 is toremove the dust adhering to the web 1, the counter-electrode 15 in theenclosure above the web is now taken to a low-voltage or groundpotential. To the opposite side of the web 1 there is placed aconductive wire 16 running over a triangular path on guide rolls 17, 18,19. With the help of a high-voltage supply 20, this conductive wire 16is now taken to a higher potential than the counter-electrode 15 in thepost-collector device enclosure. Obviously, the running wire can bereplaced by an endless belt or a band.

[0018] The field effect of the electrode tips 8 must extend over thedesired area of collection. Since the electric field shed from each ofthe electrode tips 8 has a conical shape, the number and placement ofthe electrode tips must be arranged so that a uniform field is formed onthe counter-electrode 6 by the resultant field of the conical componentfields shed from the arrayed tips of discharge electrode tips. Therequired voltage depends on the distance of the counter-electrode 6 fromthe discharge electrode tips 8 that may vary from 2 mm to 2 m, while inpractice a distance of 100 mm to 1000 mm must be used due to the spacerequired by dust collection/transfer equipment. While a greater distancebetween the electrodes as such has no effect on the function of theapparatus, it will increase the size of the apparatus. The voltageapplied between the electrodes may be varied in the range of 30-1000 kV,however, typically a voltage range of 80-160 kV has been found practicalfor the above-mentioned electrode arrangement. The counter-electrode maybe taken to a positive or negative potential and, respectively, also theelectrode tips can be connected to the positive or negative polarity ofthe voltage supply. In the collection of dry dust, the above-describedapparatus functions as follows. Web 1 is passed to the guide roll 2from, e.g., a edge-trim slitter which during the trimming step releasesdust from the edge of the web that subsequently begins to travel alongwith the moving web 1 due to the boundary air layer carried by the web.When the web 1 comes under the discharge electrode tips 8, the ion-blaststream emitted from the electrode tips 8 carries the dust particlestoward the conductive wire 6 which supports the travel of the web 1. Thedust particles will adhere to the web 1 under electric and mechanicalforces. Followingly, the dust particles will continue to travel on theweb 1, thence being prevented from being scattered about. As a permanentadhesion of the dry dust to the web is not generally desirable, the dustis subsequently removed from the web. The dust-removal step is carriedout be means of a post-collector device 13. In this apparatus, theelectrode potentials are reversed in regard to those used in theabove-described dust-adhering apparatus, whereby also the charge of thedust particles is reversed allowing them to leave the web 1. Next, thereleased dust migrates toward the counter-electrode 15 of thepost-collector device 13 and further away from the post-collector devicealong with the suction flow established by the suction fan 14. Thecollected dust may be recirculated to the web formation processor, forinstance, combusted to produce thermal energy.

[0019] In addition to the collection of dry dust, the method accordingto the invention may be employed to bind back to the web 1 either thefiber-containing water mist emitted from the web formation process, or,particularly, the coat mist emitted from the coating equipment, or,particularly, a specifically generated coat aerosol that is applied tothe web at least partially by virtue of the ion-blast technique. Such acoat aerosol can be made with the help of spray nozzles, for instance.Herein, the post-collector device 13 will obviously be omitted, becausethe particles are desiredly adhered to the web in a permanent manner.The ion-blast assembly may under certain conditions act as a capacitorcapable of storing a charge, whereby the forces that hold the webagainst its carrier can act disturbingly after the web has passed thecounter-electrode area. In order to neutralize such forces ofattraction, a corona treatment operating with positive or negative ionsmay be employed downstream from the web formation unit. The coronatreatment is carried out using a device 12 with a structure similar tothat of the ion-blast assembly.

[0020] In FIG. 2 is shown schematically a collection method for mistemitted from the nip 21 of a transfer-roll coater. As the structure ofthe transfer-roll coater is irrelevant to the application of theinvention, the coater is illustrated only for the nip 21 formed betweentwo rolls 22, 23. The lower roll is a transfer roll 22 from whosesurface the metered coat is transferred in the nip 21 to the surface ofthe web 1 passing through the nip. The function of the backing roll 23is to maintain a proper distance between the web 1 and the transfer roll22. Particularly at high web speeds, a great amount of mist 24 will beemitted from the nip of a transfer-roll coater due to the interaction ofthe web 1 and the applicator roll. The mist is formed when a portion ofthe coat film carried on the surface of the transfer roll 22 adheres tothe web 1 and the other portion continues to adhere to the surface ofthe transfer roll 22, whereby the coat film undergoes splitting when theweb 1 exits from the nip 21 and thereby some amount of coat droplets areejected from the nip tangentially with the surface of the web 1 and thesurface of the transfer roll 22.

[0021] According to the invention, the thus emitted mist can becollected with the help of the ion-blast effect on a counter-electrode26. The electrodes 25 and 26 are mounted, e.g., as shown in FIG. 2 inthe angle formed between the transfer roll 22 and the web 1 so that thepointed electrodes 25 can be used to ionize the mist emitted into thegap between the electrodes, thus transporting the mist to thecounter-electrode 22, wherefrom it can be collected for reuse.Accordingly, the electrodes are arranged so that the emitted mist willbe enclosed by the electrodes. While the coating mist falls naturallydownward along the counter-electrode surface gravitationally, itsremoval may be augmented by vibration or scraping.

[0022] Obviously, the above-described examples and the differentembodiments covered by their specifications in the appended claims maybe implemented using one or a greater number of power supplies.

1. A method of controlling of particulate matter such as mist and dustin the manufacture of paper and board, in which method a web (1) ispassed to web treatment equipment (22, 23) wherein at least onetreatment step is applied to the web (1) causing the emission of theparticulate matter (24), characterized by placing in the vicinity of aemission point (21) of said particulate matter at least two electrodes(25, 26), connecting at least one electrode,—a counter-electrode (26)—,to a low potential, connecting at least one electrode (25) to apotential higher than that of said counter-electrode (26), and makingthe potential difference between said electrodes (25, 26) so high as togenerate a corona discharge between said electrodes that causes anion-blast wind toward the electrode (26) of the lower potential, saidion-blast wind being capable of transferring particulate matter, whichenters the gap between said electrodes, toward said electrode of lowerpotential.
 2. A method as defined in claim 1, characterized byimplementing said counter-electrode bu using an endlessly movable,electrically-conducting support member (6), mounting said conductingsupport member (6) to rest against said moving web (1), and adapting onthe opposite side of the web (1) in regard to said support member (6) aplurality of pointed discharge electrodes (8) that are taken to apotential higher than that of said counter-electrode (6), wherebyparticulate matter, which enters the gap between said pointed electrodes(8) and said web (1), will migrate to said web (1) so as to adhere tothe same under electric and mechanical forces.
 3. A method as defined inclaim 1, characterized by implementing said electrode connected to ahigher potential by using an endlessly movable, electrically-conductingsupport member (16), mounting said conducting support member (16) torest against said moving web (1), and adapting on the opposite side ofthe web (1) in regard to said support member (16) at least onecounter-electrode (15) that is taken to a potential lower than that ofsaid moving support member (6), whereby particles adhered to said web(1) are detached from said web and migrate toward said counter-electrode(15).
 4. A method as defined in claim 1, in which method coat is appliedto the surface of a web (1) by means of an applicator device,characterized by arranging electrodes (26, 25) in the vicinity of themist formation area to bring the emitted mist into the gap between saidelectrodes, wherefrom the mist is transferred by the ion-blast effect tothe counter-electrode.
 5. A method as defined in any of foregoingclaims, characterized in that the potential difference between theelectrodes is selected to be in the range of 30-1000 kV, advantageously80-160 kV.
 6. A method as defined in claim 5, characterized in that thedistance between the electrodes of different potentials is set to be inthe range of 2 mm-2 m, advantageously 100-1000 mm.
 7. A method asdefined in claim 2, characterized by using treatment equipment of theweb (1) for forming coat mist which is at least partially transferred tothe web and adhered thereto by means of ion-blast techniques.
 8. Anassembly for controlling particulate matter such as mist and dust in themanufacture of paper and board, said assembly comprising means forpassing a web (1) to web treatment equipment (22, 23) wherein at leastone treatment step is applied to the web (1) causing the emission ofsaid particulate matter (24), characterized by at least two electrodes(6, 8) placed in the vicinity of the emission point of said particulatematter, at least one power supply (11) capable of taking at least oneelectrode,—a counter-electrode (6)—, to a low potential and furthertaking at least one electrode (8) to a potential higher than that ofsaid counter-electrode (6) in a manner that makes the potentialdifference between said electrodes (6, 8) so high as to generate acorona discharge between said electrodes that causes an ion-blast windtoward the electrode (6) of the lower potential, said ion-blast windbeing capable of transferring particulate matter, which enters the gapbetween said electrodes, toward said electrode of lower potential.
 9. Anassembly as defined in claim 8, characterized in that saidcounter-electrode is an endless support member (6) mounted so as totravel synchronously at least partially backing said moving web (1) andthat said electrodes taken to the higher potential are pointedelectrodes (8).
 10. An assembly as defined in claim 8, characterized inthat said electrode taken to the higher potential is an endless,electrically-conducting support member (6) mounted so as to travelsynchronously at least partially backing said moving web (1) and thatsaid counter-electrode is adapted into an enclosure taken to a vacuumand having its open side adapted to face the opposite side of the web(1) in regard to said support member (6).
 11. An assembly as defined inclaim 8, in which assembly the treatment equipment of the web (1) is acoat applicator device, particularly a transfer-roll coater (22, 23),characterized in that said electrodes (25, 26) are adapted in the angleformed between the web (1) exiting said coater (22, 23) and said coatapplicator device (22) so as to bring the mist emitted from saidapplicator device into the gap between the electrodes, wherefrom it istransferred by virtue of the ion-blast effect to the electrode of thelower potential.
 12. Use of ion-blast techniques in the coating of apaper or board web for the purpose of transferring coat particles tosaid web (1) and assuring the adherence of coat to said web (1).